Field of the Invention
The present invention relates to transcribers or tape recorders, and particularly to an automatic backspace device incorporated, for example, in a transcriber or tape recorder, and especially to an automatic backspace device wherein the magnetic tape is reliably rewound and a magnetic playback is reliably retracted from the surface of the magnetic tape even when the supply voltage fluctuates.
Conventionally, office work is enhanced by recording a letter or other document on magnetic tape and having a secretary type a draft while playing back the tape and listening to its contents.
A known transcriber allows a foot switch to start and stop the playback and also permits adjustment of the playback speed. When the typist types while listening to the recorded contents of the magnetic tape and the typing operation is appreciably slower than the playback speed, the typist must stop and pause from time to time, for example at punctuations in the draft, in order to hear the contents reliably.
However, when the playback operation is stopped for a pause, the tape may sometimes go beyond the beginning of the next sentence or phrase due to the operational delays inherent in the transcriber. Therefore, the beginning of the next sentence or phrase may be skipped when playback resumes. Therefore the next phrase or sentence can sometimes not be clearly understood. This inevitably requires the magnetic tape to be rewound a suitable length after all pauses. However, such rewind of the magnetic tape is laborious and disadvantageous.
Automatic backspace devices have been proposed to overcome these disadvantages. Such devices automatically rewind the magnetic tape a predetermined time when the playback is stopped so as to ensure understanding of the subsequent reproduced contents.
Conventional automatic backspace devices rewind the magnetic tape a predetermined amount of time, when playback is stopped, on the basis of the operation of a time constant circuit. However, such automatic backspace devices are influenced by fluctuations in the supply voltage. Such fluctuations occur most specifically in the widely available transcribers using batteries as their electrical supplies. In other words, supply voltage fluctuations may lower the voltage of the battery cell sufficiently to significantly decrease the period during which the magnetic tape is rewound. This introduces the possibility of the system not rewinding the tape completely.
Other existing automatic backspace devices use a one-shot multivibrator instead of the aforementioned time constant circuit. However, such a one-shot multivibrator is expensive. This is particularly true with respect to one-shot multivibrators using low voltage sources, such as, for example, a battery source. Therefore, these types of battery devices have the disadvantage of being difficult to manufacture from the economic point of view.
Aside from the above, rewinding a magnetic tape for a predetermined time to automatically backspace requires separating a magnetic playback head from the surface of the magnetic tape during the predetermined time period. For this purpose, proposals have been made to retract the magnetic playback head from the surface of the magnetic type electromagnetically, simultaneously with the pause instructions during playback. However, an electromagnetic arrangement may also respond to fluctuations in the voltage supply. For example, it takes a comparatively long time to move the magnetic playback head when the battery voltage that supplies the system drops. Therefore, in the worst case, a rewind may start before the magnetic playback head separates from the surface of the magnetic tape. This may result in damage to the magnetic playback head.